2. At the end of this lecture, the students will be able to: 1.Define receptors 2.Define agonists 3.List types of agonists 4. Define antagonists 5. List types of antagonists

3.  Define as any functional molecule in a cell to which a drug binds to produce its effect (target molecule for drug action).

5.  Affinity: The term affinity refers to the strength of the attraction between a drug and its receptor.  Intrinsic Activity: The term intrinsic activity refers to the ability of a drug to activate a receptor following binding.

6. Agonists: Agonists are molecules that binds to receptors and activate it. They initiates biochemical and physiological changes inside the cells that leads to drug's effect or response. Examples: Cholinergic agonists such as carbachol

7.  Antagonist bind to receptors but do not activate it. i.e. they have no effects of their own on receptor function.  They have affinity for a receptor but with no intrinsic activity.  Examples: Antihistamines: block histamine H1 receptors.

8.  Partial agonist is an agonist that has only moderate intrinsic activity. As a result the maximal effect that a partial agonist can produce is lower than that of a full agonist.  Pentazocin is an example of a partial agonist.

9.  Down regulation: When the receptors of a cell are continuously exposed to an agonist, the cell usually becomes less responsive.  Up regulation: Continuous exposure to antagonists has the opposite effect, causing the cell to become hypersensitive (supersensitive).

10. The effect of one drug or an endogenous substance is diminished or completely abolished in the presence of another drug. Types:  Chemical antagonism  Pharmacokinetic antagonism  Antagonism by receptor block  Physiological antagonism

11.  Examples: Antacids antagonize the action of Hcl in the stomach.

12.  Pharmacokinetic antagonism describes the situation in which a drug reduces the concentration of the other drug at its site of action.  Warfarin degradation is increased by drugs that accelerate its hepatic metabolism such as phenobarbitone

13.  When one drug block a receptor preventing other drug or endogenous substance from binding to this receptor.  Examples: Drugs that block adrenergic receptors such as propranolol

14.  Physiological antagonism occur when one drug antagonize the physiological action of another drug or endogenous substance.  Examples: Histamine acts on receptors of the parietal cells of the gastric mucosa to stimulate acid secretion, while omeprazole blocks this effect

15.  MEC is defined as the plasma drug level below which therapeutic effects will not occur.

16.  Therapeutic range is a plasma drug levels falling between the MEC and the toxic concentration

17.  The therapeutic index is a measure of a drug's safety. It is defined as the ratio of a drug's LD 50 to ED 50.  A large therapeutic index indicates that a drug is relatively safe.  Conversely, a small therapeutic index indicates that a drug is relatively unsafe.